Communications systems, such as a digital cordless telephone (DCT) system typically comprise one or more centralized communication stations, each of which may communicate with one or more localized stations. For instance, in a DCT system portable handheld telephone units communicate with each other either through a single central station or through inter connected parent stations. Each parent station acts as a centralized station (hereinafter referred to as a remote unit) and the portable telephone units are local units.
Transmission and reception between the remote and local units should occur at specific operating frequencies within an operation band. However, to optimize the number of communications occuring concurrently within the band, the frequencies used must remain stable over long periods of time. Typically, the transmission frequencies are generated using a reference oscillator such as a voltage controlled oscillator. Therefore, the stability of the operating frequencies depends upon the stability of the reference frequency generated by the reference oscillator. Variation of the reference frequency can result from fluctuations in temperature and aging of the oscillator.
Perhaps the simplest method for optimizing the stability of the operating frequencies is to utilize an oscillator with a very accurate reference frequency. However, the use of such oscillators may become prohibitive due to cost.
A number of frequency control systems have been developed to compensate for temperature and aging effects on the stability of the reference frequency. Most of these techniques compare the reference frequency or phase with an appropriate measure of the received signal's frequency or phase. For instance, it is known to use a phase locked loop to frequency lock the reference signal to the receive signal. However, these techniques require continuous monitoring of the reference signal which in turn requires additional hardware and software.
To minimize the cost and complexity of a system capable of compensating for changes in the frequency of a reference signal, it is desirable to use a standard VCO and eliminate the need for the aforementioned additional hardware and software.